This invention relates to removal of a coating from an article surface, and, more particularly, to removal of a dense ceramic coating from an article surface which includes at least one of Al, Ti, Cr, Zr, and their oxides.
Gas turbine engine components, typically turbine section blades, vanes, nozzles and shrouds, are designed to operate at relatively high temperatures. During operation, such components experience strenuous environmental conditions, generally because the components are exposed at the high temperatures to a gaseous atmosphere of rapidly flowing air and products of turbine engine fuel combustion. Therefore, it has been a practice in the turbine engine art to enhance component operating life by applying various types of surface protective environmental resistant coatings or coating combinations to exposed surfaces of such components. Typical widely reported coatings on alloys that include Ti and/or Cr include metallic coatings frequently including Al, ceramic coatings some forms of which are called thermal barrier coatings, and their combinations.
During operation of such gas turbine engine articles, reaction to the thermal cycling of the engine or the impact of atmosphere borne particles or their combination has been observed to cause damage to the component. Repair of the damage frequently requires replacement of the surface protective coatings, that can have metal oxides at an interface between coatings or between coatings and the substrate. One commonly used type of environmental resistant coating is a metallic coating including the element Al, the coating including an inner portion diffused into a substrate or surface of the article. A description of forms of such coatings and a method for removal from a surface portion is included in U.S. Pat. No. 5,728,227--Reverman, (patented Mar. 17, 1998).
A form of the above mentioned high temperature ceramic protective coating frequently is called a thermal barrier coating (TBC) or TBC system. Examples have been reported in such U.S. Pat. No. as 4,055,705--Stecura et al. (patented Oct. 25, 1977); U.S. Pat. No. 4,095,003--Weatherly et al. (patented Jun. 13, 1978); U.S. Pat. No. 4,328,285--Siemers et al. (patented May 4, 1982); U.S. Pat. No. 5,216,808--Martus et al. (patented Jun. 8, 1993); and U.S. Pat. No. 5,236,745--Gupta et al. (patented Aug. 17, 1993). TBC frequently is applied over a metallic environmental coating to provide a TBC system. Removal of ceramic TBC presents a different kind of problem than removal of a metallic coating, particularly when the TBC is dense and substantially non-porous to chemicals intended to penetrate the surface of the coating. For example, it is a practice to deposit a substantially fully dense TBC using the commercially available chemical vapor deposition process. Application of an acidic solution to a TBC coating surface, to react chemically with and remove the ceramic, is time consuming and, more importantly, can enlarge the size of and change cooling air flow through air cooling openings, as discussed in the above identified Reverman patent.
Removal of TBC from an article surface has included exposure of the outer surface of the TBC to a halide intended to penetrate the outer surface and then attack the interface between the TBC and the substrate. Examples of such reported methods are included in U.S. Pat. No. 4,889,589--McCormas (patented Dec. 26, 1989) and U.S. Pat. No. 5,614,054--Reeves, et al. (patented Mar. 25, 1997). However, treatment of a dense TBC, particularly a substantially fully dense TBC, using known methods can be very time consuming and, therefore, costly.